Great time of the year, for sure.
You could use your car all the way.
Driving from Kruger to Durban could take you through
the historic battlefields.
Driving from Durban to Cape Town would include some
great coastal scenery and a good look at less-developed
areas of the country. Travel during the day only.
Maybe plan your trip to fly home from Cape Town.
The long drive all the way back to Johannesburg doesn't offer much
interest - especially as the 'finale' to what would be a great trip.
Here are a few hundred web sites (many of them 'gateway sites'
to many other sites) http://www.cbel.com/south_africa/
To look at B&B's, Guest houses, etc., I suggest:
www.safarinow.com & http://www.sa-venues.com
You should expect $40+ per day for a car such as a
Nissan Almera - including the minimum recommended insurance.